1. Introduction
This invention relates to the formation of through-hole and multilayer printed circuit boards using an epoxy printed circuit board base material, and more particularly, to an electroless metal plating process for the fabrication of such boards employing a pretreatment sequence including contact with an activated permanganate solution.
2. The prior art
When preparing printed circuit boards having more than one circuit pattern, conductive holes, known in the art as through-holes, pass through the board to accommodate insertion and soldering of electrical component leads and to make electrical connections between two or more circuit patterns. Conductive through-holes are conventionally introduced by drilling or punching holes through a copper clad, rigid board followed by a plating procedure. The holes usually plated by a copper reduction procedure known in the art as electroless copper plating such as disclosed by Coombs, Jr., "Printed Circuits Handbook", McGraw-Hill Book Co., N.Y., N.Y., 1967, chapter 5 and by Draper, "Printed Circuits and Electronic Assemblies", Robert Draper, Ltd. Teddington, 1969, Chapter 6, both incorporated herein by reference. The copper clad board with plated through-holes can be processed into printed circuit boards using resists and processes such as those disclosed in the aforementioned "Printed Circuit Handbook" or, for example, in any of U.S. Pat. Nos. 3,469,982; 3,526,504; 3,547,730; 3,622,344 and 3,837,860, all incorporated herein by reference. As is known in the art, the copper reduction process involves pretreatment with a catalytic solution catalytic to the deposition of an electroless metal such as a tin-palladium colloid, followed by electroless metal plating, typically an electroless copper deposition solution. Processes of this nature are disclosed in U.S. Pat. No. 3,011,920 incorporated herein by reference.
Printed circuit board failure often occurs as a consequence of poor adhesion of the metal plated onto the sidewalls of the through holes. There are several causes of poor adhesion. For example, when a through-hole is drilled through the board, a substantial temperature increase occurs at the interface between the drill bit and the wall of the through hole as a consequence of friction. This results in melting of the plastic substrate at its interface with the drill bit. The melted plastic on the surface of the hole wall forms in a smooth smear over the surface of the hole which is poorly adsorptive of plating catalyst. Another cause of poor adhesion is the presence of minute particles of the plastic formed during the drilling or punching operation. These particles can interfere with the bond between the metal plate and the wall of the through hole. When a circuit board base material comprising glass filled epoxy is used as the circuit board base material, the fibrils of the glass fiber often protrude into the hole and interfere with the bond between the subsequently deposited metal and the side wall of the hole.
An advance in the art of printed board manufacture was the introduction of the multilayer board where multiple circuits are stacked in layers on a circuit board and pressed into a multilayer board. Each circuit is separated from another by a layer of a dielectric material. In a manner similar to the double sided through-hole circuit board, holes are formed in the multilayer stack to form interconnections between the same.
Adhesion of copper to sidewalls of through-holes is more critical in the manufacture of multilayer boards than in the manufacture of the two sided through-hole board. However, because there are a stack of multiple circuits, through-hole plating is more difficult than through-hole plating of double sided boards and good and consistent adhesion of the copper to the sidewall is more difficult to achieve.
Another problem in the manufacture of multilayers is epoxy smear over the copper of the copper circuit protruding into the through-hole. For electrical contact between the inner layer circuit and the metal on the hole wall, there must be contact between the two. The epoxy smear often acts as a dielectric preventing this contact. Consequently, treatments such as sulfuric acid or chromic acid etching or plasma etching have been used to remove this smear. However, these treatments smooth the hole-walls further increasing the problem of adhesion of a metal plate to the sidewall of the hole.